Because of the increased interest in protecting digital data from illegal copying, watermarking of data has, in recent years, become increasingly popular. Embedding a watermark into a digital data file involves selecting samples from the digital data file and recording in selected bits of these samples, data comprising copyright information. Arrangements can then be made so that any unauthorized access or copying of the original data file runs the risk of the extracted watermark exposing the lack of legal ownership of the file.
An official and reliable watermark has to be difficult to find and remove or override. In addition, the watermark should not affect substantially the quality of the original data file.
Copyright protection by way of watermarking has become especially popular in the music industry, where recently there has been a strong increase in illegal downloads and copying. For the process of watermarking audio files, however, an additional consideration is related to the fact that many such files, together with the embedded watermarks, are processed on mobile phones and other hand-held players. In order to minimize cost and extend battery life, such hand-held devices often have slow processors with limited computational capabilities.
Some of the methods developed for embedding watermarks use spread spectrum techniques in the frequency domain. These methods generally require the original audio data for watermark detection. These methods also are computationally intense, because of the complex transformations involved in the data processing. Accordingly, these methods are not suitable for processing watermarked files in hand-held devices.
Other techniques embed watermarks in the time domain. Many of these techniques use the least significant data bits of the respective data samples to store the watermark. One disadvantage of this approach is that the stored watermark can be erased without significantly eroding the audio quality, thus undermining the reliability of the protection.
Accordingly, it is desirable to develop a method for embedding a watermark in digital data that is tamper-resistant and relatively simple, so that the verification of the watermark would not require substantial computational capabilities.